1. Field of the Invention
The present invention relates to a steel sheet used for structural components and functional components of automobiles, particularly to a steel sheet that provides high strength and superior toughness by applying hardening treatment thereto after forming thereof to component shapes, and also relates to a method for manufacturing the same.
2. Description of Related Arts
High strength steel sheets having a strength of 980 MPa or more are currently applied to structural components of automobiles, including door impact beams and center pillars, taking advantages of light weight and durability thereof. These steel sheets are also used as functional components of automobiles, including gear parts for seat recliners and window regulators, owing to the excellent wear resistance.
Those components require, however, sever formability, so the current steel sheets often raise problems of cracks and imprecision of dimension. Furthermore, those types of steel sheets are expensive as base materials.
With that kind of problems, there have been trials in recent years to use steel sheets having 440 MPa level strength for further increasing the strength thereof by applying heat treatment such as induction hardening treatment after formed to respective components. For example, [MATERIA, vol.37, No.6 (1998)] discloses an example of strengthening steel sheets having 440 and 390 MPa of original strength, respectively, by forming them to center pillar re-enforcement and to front cross member, followed by applying induction hardening treatment. The disclosure proposes a method of induction hardening treatment, in which a heating coil supported by a robot is moved precisely along the profile of the component to minimize the variations in hardening treatment conditions.
JP-A-60-238424 and JP-A-7-126807, (the term xe2x80x9cJP-Axe2x80x9d referred herein signifies the xe2x80x9cUnexamined Japanese Patent Publicationxe2x80x9d), disclose a method for increasing the strength, in which the hardening treatment is conducted by irradiating high energy density beam such as laser to a formed component.
According to the method disclosed in [MATERIA, vol.37, No.6 (1998)], however, a specially designed robot is required to decrease the variations in the hardening treatment conditions, which induces significant increase in the investment cost.
According to the method disclosed in JP-A-60-238424 and JP-A-7-126807, the zone of irradiation of high energy density beam is narrow, and a long time is required to strengthen the total target component, which significantly degrades the productivity and increases the investment cost. The steel sheets receiving those kinds of treatment give around 710 MPa of strength, at the highest, after hardening treatment, and also give not sufficient toughness. Therefore, these steel sheets cannot be applied to the structural components such as door impact beams and center pillars, and to the functional components such as gear parts used in seat recliners and window regulators, all of which require strengths of 980 MPa or more.
An object of the present invention is to provide a steel sheet that increases the strength to a level necessary for structural components and functional components of automobiles by applying an inexpensive hardening treatment method, while giving excellent toughness after hardening treatment, and to provide a method for manufacturing the same.
The object is attained by a steel sheet that consists essentially of 0.10 to 0.37% C, 1% or less Si, 2.5% or less Mn, 0.1% or less P, 0.03% or less S, 0.01 to 0.1% sol.Al, 0.0005 to 0.0050% N, 0.0003 to 0.0050% B, by mass, and balance of Fe, [14B/(10.8N)] being 0.5 or more, average particle size of precipitate BN being 0.1 xcexcm or more, and grain size of prior austenite after hardening treatment being 2 to 25 xcexcm.
The above-described object is attained also by a steel sheet, further adding Ti to the composition of the above-described steel sheet, which consists essentially of 0.10 to 0.37% C, 1% or less Si, 2.5% or less Mn, 0.1% or less P, 0.03% or less S, 0.01 to 0.1% sol.Al, 0.0005 to 0.0050% N, 0.005 to 0.05% Ti, 0.0003 to 0.0050% B, by mass, and balance of Fe, [Bxe2x88x92(10.8/14)N*] being not less than 0.0005%, average particle size of precipitate TiN being 0.06 to 0.30 xcexcm; grain size of prior austenite after hardening treatment being 2 to 25 xcexcm, N* being set to zero for negative N* values calculated in a definition formula of N*=[Nxe2x88x92(14/48)Ti].
These steel sheets can be manufactured by a method for manufacturing a steel sheet, which has the steps of: hot rolling a steel slab consisting essentially of 0.10 to 0.37% C, 1% or less Si, 2.5% or less Mn, 0.1% or less P, 0.03% or less S, 0.01 to 0.1% sol.Al, 0.0005 to 0.0050% N, 0.0003 to 0.0050% B, by mass, and balance of Fe, [14B/(10.8N)] being 0.5 or more, or essentially of 0.10 to 0.37% C, 1% or less Si, 2.5% or less Mn, 0.1% or less P, 0.03% or less S, 0.01 to 0.1% sol.Al, 0.0005 to 0.0050% N, 0.005 to 0.05% Ti, 0.0003 to 0.0050% B, by mass, and balance of Fe, [Bxe2x88x92(10.8/14)N*]] being not less than 0.0005%, at temperatures of Ar3 transformation point or above; and coiling thus hot rolled steel sheet at temperatures of 500 to 720xc2x0 C.